Balanced magnetic transducer

ABSTRACT

The transducer includes a cup-shaped magnetic enclosure having a center pole piece threaded into the base thereof. A coil is disposed about the pole piece and an annular permanent magnet encircles the coil, the magnet being in engagement with the base but spaced from the wall of the enclosure. An annular pole piece is mounted on the magnet and a magnetic armature overlies both the center and magnet pole pieces, a nonmagnetic support locating the armature in close proximity with the pole pieces and spaced from the wall of the enclosure. An annular balancing pole piece secured to the wall of the enclosure extends into close proximity with the upper surface of the armature to complete the magnetic circuit.

United States Patent [72] Inventor Melville S. llawley Indianapolis, Ind. (21] Appl. No 781,854 [22] Filed Dee. 6,1968 [45] Patented June 8, I971 [73] Assignee Bell Telephone Laboratories, Incorporated Murray Hill, Berkeley Heights, NJ.

[54] BALANCED MAGNETIC TRANSDUCER 9 Claims, 3 Drawing Figs.

U.S.Cl. 179/119R Assistant ExaminerThomas L. Kundert Attorneys-R. J. Guenther and Edwin B. Cave ABSTRACT: The transducer includes a cup-shaped magnetic enclosure having a center pole piece threaded into the base thereof. A coil is disposed about the pole piece and an annular permanent magnet encircles the coil, the magnet being in engagement with the base but spaced from the wall of the enclosure. An annular pole piece is mounted on the magnetand a magnetic armature overlies both the center and magnet pole pieces, a nonmagnetic support locating the armature in close proximity with the pole pieces and spaced from the wall of the enclosure. An annular balancing pole piece secured to the wall of the enclosure extends into close proximity with the upper surface of the armature to complete the magnetic circuit.

BALANCED MAGNETIC TRANSDUCER FIELD OF THE INVENTION This invention relates to the field of communications and within that field to electroacoustic transducers for converting sound waves into electrical signals wherein the transducers serve as transmitters, and for converting the electrical signals into sound waves wherein the transducers serve as receivers.

DISCUSSION OF THE PRIOR ART As indicated in US. Pat. No. 2,360,796 issued to J. S. P. Roberton on Oct. 17, 1944, it is recognized in the electroacoustic transducer art that there are several advantages to a balanced armature system, a balanced armature system being one in which two criteria are met. First, the armature is positioned between oppositely magnetized poles, and the polarizing flux passing through the airgaps on each side of the armature acts upon the armature with equal force. Second, the magnetic circuit is such that when the signal flux aids the polarizing flux in .the airgaps on one side of the armature, it opposes the polarizing flux in the airgaps on the other side of the armature. Since the pull on one side of the armature in creases at the same time that the pull on the other side of the armature decreases, the balanced armature system provides both very high efficiency and reduced distortion due to harmonics.

However, a balanced armature system also has some disadvantages. As exemplified by the aforementioned patent, in many balanced armature systems the armature serves as the diaphragm or is part of the diaphragm of the electroacoustic transducer, and then the diaphragm must be clamped at its periphery. Otherwise it is unstable because of the equal magnetic forces acting to deflect it in opposite directions. Clamped diaphragms, though, tend to distort after repeated expansions and contractions due to normal temperature fluctuations. This distortion results in a displacement of the armature from its balanced position and therefore adversely affects the response of the transducer.

In addition, as also exemplified by the aforementioned patent, balanced armature systems are more complex than unbalanced systems in that they have several permanent magnets and/or several coils and/or a linkage between the armature and the diaphragm. Furthermore, they have to be accurately formed and assembled to provide the desired balance. All of these factors result in balanced armature systems being more expensive than unbalanced systems.

SUMMARY OF THE INVENTION An electroacoustic transducer is herein disclosed that has a pseudobalanced armature structure so that it approaches the high efficiency of such a structure but at the same time does not have the mechanical instability and the high cost of fabrication of the true balanced armature structure. Specifically, the electroacoustic transducer herein disclosed has the characteristics of a balanced armature system except that the force exerted by the polarizing flux in the airgaps on one side of the armature does not equal the force exerted by the polarizing flux in the airgaps on the other side of the armature. This arrangement does not have as high an efficiency or reduce harmonics to as great a degree as in a completely balanced armature system, but it does permit the use of an unclaniped diaphragm and it does allow for a simpler and less expensive structure.

Thus the electroacoustic transducer of the present invention comprises a permanent magnet and two pole pieces positioned on one side of the armature and a balancing pole piece positioned on the other side of the armature. Each of the pole pieces is magnetically connected to an individual pole of the permanent magnet, the two pole pieces on the one side being magnetically connected to different poles of the permanent magnet. In addition, one of these two pole pieces has a coil disposed thereabout, and it is connected to the same pole of the permanent magnet as the balancing pole piece.

With this arrangement the pole pieces exert a magnetic attraction on both sides of the armature, but there is a greater force exerted by the two pole pieces on the one side than the balancing pole piece exerts on the other side. The difference between the two forces maintains the armature in position. Furthermore, with this arrangement the flux paths are such that the signal flux aids the polarizing flux on one side of the armature and opposes the polarizing flux on the other side of the armature.

DESCRIPTION OF THE DRAWING FIG. I is a cross-sectional view of an embodiment of the electroacoustic transducer of this invention;

FIG. 2 is a schematic view of the electroacoustic transducer of FIG. 1 showing the polarizing flux paths; and

FIG. 3 is a schematic view of the electroacoustic transducer of FIG. 1 showing the signal flux paths.

DETAILED DESCRIPTION Referring to FIG. 1 of the drawing, the electroacoustic transducer of this invention includes a cylindrical cup-shaped support member 10 formed from a soft magnetic material, the support member 10 having a base portion 12 and a wall portion 14. The wall portion I4 has a step therein to provide a shoulder 16, and the base portion 12 has a centrally located cylindrical hub 18 that is threaded internally to accommodate a center pole piece 20.

The center pole piece 20 is a cylindrical member that is externally threaded along the lower portion of its length. The upper end of the center pole piece 20 is provided with a pole face 22 while the lower end is provided with a slot 24, the slot being engageable by a screwdriver to adjust the position of the center pole piece within the hub 18.

A coil 25 is disposed about the center pole piece 20, the coil being wound on a spool (not shown), and an annular permanent magnet 30 encircles the coil. The permanent magnet 30 is coaxial with the center pole piece 20, and the permanent magnet is magnetized axially so that one pole is at its upper end and the other pole is at its lower end. The permanent magnet 30 is mounted with its lower end, which in this case is the south pole, in engagement with the base portion 12 of the support member 10 while the upper end of the permanent magnet, which in this case is the north pole, has an annular pole piece 35 mounted thereon.

The pole piece 35, which is L-shaped in axial cross section, serves to concentrate the polarizing flux flowing from the upper end of the permanent magnet 30 and is therefore properly referred to as the magnet pole piece. The magnet pole piece 35 has a pole face 36 that is advantageously located in approximately the same plane as the pole face 22 of the center pole piece 20, and both the permanent magnet 30 and the magnet pole piece are spaced from the wall portion 14 of the support member 10 so that little flux leakage occurs between the sides thereof and the wall portion.

A diaphragm assembly 40 is positioned on the shoulder 16 of the wall portion 14, the diaphragm assembly comprising a diaphragm 42 and an armature 45. The diaphragm 42 is a flat nonmagnetic disc of a diameter slightly less than the inside diameter of the upper end of the wall portion 14, and the diaphragm has a central opening therein. The armature 45 is a flat, soft magnetic disc of greater diameter than the central opening in the diaphragm 42 and the armature is bonded to the upper surface of the diaphragm concentric with the opening therein. The elements of the electroacoustic transducer are so shaped and sized that the diaphragm 42 supports the armature 45 in close proximity with the pole faces 22 and 36 of the center and magnet pole pieces 20 and 35 but substantially spaced from the wall portion 14 of the support member 10.

The electroacoustic transducer structure is completed by an inverted cylindrical cup-shaped cover member 50 having a wall portion 52 and a top portion 54. The wall portion 52 is of a diameter to be telescoped over the wall portion 14 of the support member and is of a length to be swaged over the shoulder 16 of the wall portion 14 to join the cover member 50 to the support member. The top portion 54 of the cover member 50 has a hole 58 therein and circumscribing the hole is a pole piece 60 that extends downward into close proximity with the armature 45. The hole 58 permits acoustical energy generated by the diaphragm assembly 40 to be transmitted external to the transducer when the transducer is used as a receiver and permits acoustical energy generated external to the transducer to impinge upon the diaphragm assembly when the transducer is used as a transmitter.

The cover member 50, like the support member 10, is formed from a soft magnetic material, and thus with the cover member joined to the support member the pole piece 60 is magnetically connected to the permanent magnet 30. The term magnetically connected" as used in the specification and in the claims means joined by an essentially continuous ferromagnetic path. Furthermore, the cover member 50 and the support member 10 combine to form a magnetic enclosure that shields the transducer from stray magnetic flux external thereto and prevents the magnetic flux generated by the transducer from interfering with components adjacent thereto.

Referring now to FIG. 2, with the foregoing structure the polarizing flux generated by the permanent magnet 30 flows from the north pole of the permanent magnet through the magnet pole piece 35, through the airgap between the magnet pole piece and the armature 45, and into the armature. At this point, the polarizing flux divides, part of the polarizing flux flows longitudinally through the armature 45 to its center, through the airgap between the armature and the center pole piece 20, through the center pole piece, and then through the support member 10 to the south pole of the permanent magnet 30. The other portion of the polarizing flux flows transversely through the armature 45, through the airgap between the armature and the pole piece 60, and then through the cover and support members 50 and 10 to the south pole of the permanent magnet. These two polarizing flux paths produce permanent polarities at the gaps as shown with capital letters in FIG. 2.

As a result of these two polarizing flux paths the center pole piece 20 and the magnet pole piece 35 exert a downward force on the armature 45, while the pole piece 60 exerts an upward force on the armature. Since the upward force exerted by the pole piece 60 acts to balance the downward force of the center and magnet pole pieces 20 and 35, the pole piece 60 is properly referred to as a balancing pole piece. However, the upward force exerted by the balancing pole piece 60 is not as great as the downward force exerted by the center and magnet pole pieces 20 and 35, and this difference in magnetic force maintains the armature assembly 40 (FIG. 1) seated on the shoulder 16 of the support member 10.

Turning now to FIG. 3, signal flux generated by the coil when the transducer is used as a receiver and by the movement of the armature 45 when the transducer is used as a transmitter flows through similar flux paths. The electrical signal flowing through the coil 25 or the acoustical signal impinging upon the armature 45 is of course an alternating one and consequently the signal flux is constantly reversing direction. During some portions of the signal, the signal flux flows from the upper end of the center pole piece 25, through the airgap between the center pole piece and the armature 45, and into the armature where it divides. One part of the signal flux flows longitudinally out from the center of the armature 45, through the airgap between the armature and magnet pole piece 35, through the magnet pole piece and the permanent magnet 30, and through the support member 10 back to the center pole piece 20. The other part of the signal flux flows transversely through the armature 45, through the airgap between the armature and the balancing pole piece 60, and through the cover and support members 50 and 10 back to the center pole piece 20. During the other portions of the signal, the signal flux flows in the opposite direction.

The signal flux flow during the first of the above-mentioned portions of the signal produces signal polarities at the gaps as shown with small letters in H0. 3. As seen by comparing the signal polarities with the permanent polarities in FlG. 2, the signal flux aids the polarizing flux in the gap between the armature 45 and the balancing pole piece 60, but opposes the polarizing flux in the gaps between the armature and the center pole piece 20 and between the armature and the magnet pole piece 35. Thus the upward pull on the armature 45 increases while the downward pull decreases, and both changes act to deflect the armature upward.

When the signal flux reverses direction, the signal polarities reverse and then the signal flux aids the polarizing flux in the airgaps between the armature 45 and the center and magnet pole pieces 20 and 35 and opposes the polarizing flux in the airgap between the armature and the balancing pole piece 60. As a result, the downward pull on the armature 45 increases while the upward pull decreases, and both changes act to deflect the armature downward. The deflection of the armature whether up or down is of course proportional to the strength of the signal.

It is to be noted that with the present arrangement only a portion of the signal flux must flow through the permanent magnet 30. A permanent magnet has a high AC reluctance, and it therefore attenuates the signal flux. Thus by providing a signal flux path that shunts the permanent magnet 30, the efficiency of the electroacoustic transducer is increased.

Although a specific embodiment of the invention has been shown and described, it will be understood that it is but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What I claim is 1. An electroacoustic transducer comprising:

an armature of magnetic material;

a permanent magnet for generating a polarizing flux;

a first major polarizing flux path including a first pole piece extending adjacent to one side of the armature and magnetically connected to a first pole of the permanent magnet, an airgap between the first pole piece and the armature, and a longitudinal portion of the armature; and

a second major polarizing flux path including a second pole piece extending adjacent to the other side of the armature and magnetically connected to the first pole of the permanent magnet, the second magnetic flux path shunting the airgap and the pole piece of the first magnetic path.

2. An electroacoustic transducer as in claim 1 further including a third pole piece extending adjacent to the armature and magnetically connected to the second pole of the permanent magnet, the third pole piece being positioned between the permanent magnet and the armature, the third pole piece further being included in both the first and second major polarizing flux paths and directing the majority of the polarizing flux in a single flux path to the armature.

3. An electroacoustic transducer as in claims I, or 2 wherein the magnet magnetic connection between the second pole piece and the permanent magnet in combination with the magnetic connection between the first pole piece and the permanent magnet provides a magnetic enclosure for the transducer and the armature is supported so that the major flux flow therethrough is directed to the magnetic enclosure only through the first and second pole pieces.

4. An electroacoustic transducer as in claim 1 wherein the magnetic connection between the second pole piece and the permanent magnet in combination with the magnetic connection between the first pole piece and the permanent magnet provides a magnetic enclosure for the transducer and a third pole piece magnetically connected to the second pole extends adjacent to the one side of the armature, the third pole piece being spaced from the magnetic enclosure so that little flux flow occurs from the third pole piece directly to the magnetic enclosure.

5. An electroacoustic transducer as in claims 1, 2 or 4 further including means for generating a signal flux associated with the first pole piece and further including a major signal flux path including the airgap between the first pole piece and the armature, a portion of the armature, an airgap between the armature and the second pole piece, and the second pole piece.

6. An electroacoustic transducer comprising:

a magnetic enclosure including a base, a wall upstanding from the base, and a top overlying the base;

a balancing pole piece depending from and magnetically connected to the top;

a center pole piece upstanding from and magnetically connected to the base;

a coil disposed about the center pole piece;

a permanent magnet positioned between the coil and the wall of the enclosure, the permanent magnet being spaced from the wall but magnetically connected to the base of the enclosure;

a magnetic armature; and

a nonmagnetic support for the armature, the support locating the armature intermediate and in close proximity with the balancing pole piece and the center pole piece, but the armature from the wall of the enclosure. 8c 0250 7. An electroacoustic transducer as in claim 6 further in cluding a third pole piece positioned on the permanent magnet, the third pole piece being spaced from the wall of the magnetic enclosure but extending into close proximity with the armature.

8. An electroacoustic transducer comprising:

an armature of magnetic material;

a permanent magnet for generating a polarizing flux;

a first polarizing flux path including a first pole piece extending adjacent to one side of the armature and magnetically connected to a first pole of the permanent magnet, an airgap between the first pole piece and the armature,

and a longitudinal portion of the armature;

a second polarizing flux path including a second pole piece extending adjacent to the other side of the armature and magnetically connected to the first pole of the permanent magnet, the second magnetic flux path shunting the airgap and the pole piece of the first magnetic path; and

a third pole piece extending adjacent to the armature and magnetically connected to the second pole of the permanent magnet, the third pole piece being positioned between the permanent magnet and the armature and directing the polarizing flux in a single major flux path to the armature.

9. An electroacoustic transducer comprising:

an armature of magnetic material;

a permanent magnet for generating a polarizing flux;

a first polarizing flux path including a first pole piece extending adjacent to one side of the armature and magnetically connected to a first pole of the permanent magnet, an airgap between the first pole piece and the armature, and a longitudinal portion of the armature; and

a second polarizing flux path including a second pole piece extending adjacent to the other side of the armature and magnetically connected to the first pole of the permanent magnet, the second magnetic flux path shunting the airgap and the pole piece of the first magnetic path;

the magnetic connection between the second pole piece and the permanent magnet in combination with the magnetic connection between the first pole piece and the permanent magnet providing a magnetic enclosure for the transducer, and the armature being supported so that the major flux flow therethrough is directed to the magnetic enclosure only through the first and second pole pieces. 

1. An electroacoustic transducer comprising: an armature of magnetic material; a permanent magnet for generating a polarizing flux; a first major polarizing flux path including a first pole piece extending adjacent to one side of the armature and magnetically connected to a first pole of the permanent magnet, an airgap between the first pole piece and the armature, and a longitudinal portion of the armature; and a second major polarizing flux path including a second pole piece extending adjacent to the other side of the armature and magnetically connected to the first pole of the permanent magnet, the second magnetic flux path shunting the airgap and the pole piece of the first magnetic path.
 2. An electroacoustic transducer as in claim 1 further including a third pole piece extending adjacent to the armature and magnetically connected to the second pole of the permanent magnet, the third pole piece being positioned between the permanent magnet and the armature, the third pole piece further being included in both the first and second major polarizing flux paths and directing the majority of the polarizing flux in a single flux path to the armature.
 3. An electroacoustic transducer as in claims l, or 2 wherein the magnet magnetic connection between the second pole piece and the permanent magnet in combination with the magnetic connection between the first pole piece and the permanent magnet provides a magnetic enclosure for the transducer and the armature is supported so that the major flux flow therethrough is directed to the magnetic enclosure only through the first and second pole pieces.
 4. An electroacoustic transducer as in claim 1 wherein the magnetic connection between the second pole piece and the permanent magnet in combination with the magnetic connection between the first pole piece and the permanent magnet provides a magnetic enclosure for the transducer and a third pole piece magnetically connected to the second pole extends adjacent to the one side of the armature, the third pole piece being spaced from the magnetic enclosure so that little flux flow occurs from the third pole piece directly to the magnetic enclosure.
 5. An electroacoustic transducer as in claims 1, 2 or 4 further including means for generating a signal flux associated with the first pole piece and further including a major signal flux path including the airgap between the first pole piece and the armature, a portion of the armature, an airgap between the armature and the second pole piece, and the second pole piece.
 6. An electroacoustic transducer comprising: a magnetic enclosure including a base, a wall upstanding from the base, and a top overlying the base; a balancing pole piece depending from and magnetically connected to the top; a center pole piece upstanding from and magnetically connected to the base; a coil disposed about the center pole piece; a permanent magnet positioned between the coil and the wall of the enclosure, the permanent magnet being spaced from the wall but magnetically connected to the base of the enclosure; a magnetic armature; and a nonmagnetic support for the armature, the support locating the armature intermediate and in close proximity with the balancing pole piece and the center pole piece, but the armature from the wall of the enclosure. 8c 0250
 7. An electroacoustic transducer as in claim 6 further including a third pole piece positioned on the permanent magnet, the third pole piece being spaced from the wall of the magnetic enclosure but extending into close proximity with the armature.
 8. An electroacoustic transducer comprising: an armature of magnetic material; a permanent magnet for generating a polarizing flux; a first polarizing flux path including a first pole piece extending adjacent to one side of the armature and magnetically connected to a first pole of the permanent magnet, an airgap between the first pole piece and the armature, and a longitudinal portion of the armature; a second polarizing flux path including a second pole piece extending adjacent to the other side of the armature and magnetically connected to the first pole of the permanent magnet, the second magnetic flux path shunting the airgap and the pole piece of the first magnetic path; and a third pole piece extending adjacent to the armature and magnetically connected to the second pole of the permanent magnet, the third pole piece being positioned between the permanent magnet and the armature and directing the polarizing flux in a single major flux path to the armature.
 9. An electroacoustic transducer comprising: an armature of magneTic material; a permanent magnet for generating a polarizing flux; a first polarizing flux path including a first pole piece extending adjacent to one side of the armature and magnetically connected to a first pole of the permanent magnet, an airgap between the first pole piece and the armature, and a longitudinal portion of the armature; and a second polarizing flux path including a second pole piece extending adjacent to the other side of the armature and magnetically connected to the first pole of the permanent magnet, the second magnetic flux path shunting the airgap and the pole piece of the first magnetic path; the magnetic connection between the second pole piece and the permanent magnet in combination with the magnetic connection between the first pole piece and the permanent magnet providing a magnetic enclosure for the transducer, and the armature being supported so that the major flux flow therethrough is directed to the magnetic enclosure only through the first and second pole pieces. 